MA state building codes have requirements such as being a minimum amount of square footage, the room must have a closet, an egress, the living area needs to be above grade, etc. etcâ€¦ Each TOWN building code is different so it is imperative you inquire direct with them.
If your property has a septic system, the size and capacity of the system has to be in-line with the number of bedrooms. Also likely to be the same if you have well water. For example, if you are converting from a 3 bedroom home to a 4, and your septic system was designed for a 3 bedroom, than you will likely have to expand that system too. You will want to inquire not only with the building department but also the Board of Health as well as Conservation depending on the vegetation around you. You may even need to consult with various other departments. Ask a lot of questions....